AFAIK the Cataphract has a standard spear doing STR+3 damage, non AP (why would it be AP ?)
The reasoning is that the Serpent doesn't charge, so the rider is only mounted infantry, it does not have the special 1-shot 20 damage Lance attack of heavy cavalries.
The real boost with Serpent Cult to Cataphracts is that the Cataphract gets Sacred (and the Hydras too !
)